Amid growing standoff with Russia over Ukraine, the US is sending a guided missile cruiser to the Black Sea to reassure American allies in the region in the wake of the crisis.

"I can confirm the Vella Gulf, a Navy cruiser, will be going in to the Black Sea probably later this week," Pentagon Press Secretary Rear Admiral John Kirby told reporters.

Vella Gulf will replace frigate USS Taylor, which departed the region recently. Another NATO vessel is already in the area.

"We've got ships that are coming and going in and out of the Black Sea. This is all going to be a persistent rotational basis. That's what our focus is on," Kirby said.

He referred to the remarks of the US European Command Commander, Philip Mark Breedlove, who suggested that perhaps NATO should have discussions about more permanent US presence or additional US presence on a permanent basis.

"That's a discussion for NATO heads of state and minister of defense to have. And that discussion just hasn't happened yet," he said.

"Right now, the Pentagon's focus is on a rotational basis, training and presence that can be constantly updated," Kirby said.

"Our focus now on reassuring our allies and partners, specifically our NATO allies, with respect to this Ukraine crisis, is through a persistent rotational presence. That's what you're seeing us do. We've got members of the 173rd are still doing exercises in Poland and the three Baltic countries. We've got other exercises that are going on or will be going on soon," he said in response to a question.

The US, he said, already has tens of thousands of Americans stationed in Europe, at the request and invitation and support of many host nations.

"We are grateful for that support," he said, adding that the US is definitely committed to keeping a rotational presence going through the end of this calendar year.

"General Breedlove has said that. (US Defense) Secretary (Chuck) Hagel has said that. We're committed to that. And then we'll see where we go. At the end of the year, we'll just see where things are," he said.

The US is definitely committed to doing that through the end of this calendar year.

"That will include naval presence in and out of the Black Sea. But that's also not unusual, just the Black Sea piece. It's not uncommon for the US Navy to operate ships in and out of the Black Sea. We do it all the time," he argued.